WASHINGTON — The threat of mass shootings is the defining fear for the generation that has grown up in the shadow of Columbine, a new USA TODAY/Ipsos Poll finds. Now more than one in three young people nationwide say they plan to join the March For Our Lives protests on Saturday in person or via social media.

The survey of 13- to 24-year-olds — including more than 600 middle-school and high-school students — shows both the depth of anxiety that school violence has fueled and the way a movement has spread across the country in the weeks since a rampage at Marjory Stoneman Douglas High School in Parkland, Fla., left 17 people dead.

On Tuesday, another school shooting in Great Mills, Md., left the alleged shooter dead and two other students wounded.

“I watch over my shoulder because you never know,” says Justin McDonnall, 17, a sophomore at North Central High School in Hymera, Ind., who was among those polled. Even in his small town, which he describes as being in “Nowhere USA”, police officers spent two days at his school to deal with verbal threats of gun violence that a fellow student had made. With the marches, he said, “We’d like to be heard, and not just ignored.”

Eighteen percent of the young people polled, including 21% of those 13 to 17, say they will participate personally in the marches. If they do, it would mean the most massive student-led protests in American history, dwarfing even the anti-war demonstrations of the Vietnam era. Another 24% say they will participate using social media.

"I think the protesting is ... really great because it's showing younger kids that you need to stand up for what you believe in," Madeline Meyers, 14, an eighth-grader at Nikolay Middle School in Cambridge, Wis., said in a follow-up phone interview. "If you believe that armed teachers is not the answer, if you believe that guns in school is not the answer, then you need to show that."

The USA TODAY/Ipsos Poll is unusual because it included not only those 18 and older but also those from 13 to 17. Parents were required to give their permission before the minors could participate.

“The March for Our Lives and #NeverAgain movements have been organized and powered by young people, but before now we have not known how all the youth of our country feel about gun-control issues,” says Cliff Young, president of the polling firm Ipsos. The new survey "helps us hear their voices and understand they are tired of waiting for us to protect them.”

For the survey, 1,112 young people were interviewed online between March 14th and 20th, including 605 who were 13 to 17 and another 507 who were 18 to 24. The poll has a credibility interval of plus or minus 3.4 percentage points for the full sample, 4.5 points for the sample of those under 18, and 5 points for those 18 to 24.

There's a debate over precisely when the Millennial generation ends and the post-Millennial generation begins. There's no official authority; widely accepted definitions evolve over time. The influential Pew Research Center this month said it would define post-Millennials as those born from 1997 on, which would make them 11 to 22 years old now — similar to but not exactly the same as this poll.

Those surveyed put gun violence/crime at the top of the list of topics they find most worrying. The fear is more pronounced among those under 18: 53% in the younger age group cite gun violence as a major worry, compared with 32% of those in the older group. For both, the category outranked every other concern, including terrorism, racism, college affordability and climate change.

Since these young people enrolled in elementary school, mass shootings have been a reality. The Columbine High School massacre in Colorado was in 1999, when the oldest in this group, the 24-year-olds, were just five years old. The youngest were born six years after Columbine.

Now nearly one in five, 19%, say they don't feel safe at their school. One in four, 25%, say it's very or somewhat likely that a classmate will bring a gun to school. Nearly one in seven, 15%, say it's likely there will be a shooting at their school.

Their parents also see the threat as real. Eight in 10 of those under the age of 18 say their parents or guardians have had a "serious talk" with them about dealing with a gun in school. For those 18 to 24, almost four in 10 have had that talk. Among both groups, by an overwhelming 10-1 they say schools should be required to have drills to prepare students and faculty members for the possibility of a mass shooting.

"It is a pretty big experience because I usually imagine myself as if it was a real-life situation," Cornelius Collie, 13 years old and a seventh grader, said of the active-shooter drills at West Tallahatchie High School in Tutwiler, Miss. "Some other kids take it as a joke but I take it seriously because you never know what could happen. Like in Florida, I bet no one took it seriously until it happened."

The students surveyed support taking steps to make their schools safer. Seven in 10 say schools should be required to have an armed police officer on site. Six in 10 say schools should be required to have metal detectors at the door; more than two-thirds of those under 18 felt that way.

But the idea of training and arming teachers to use guns, a proposal backed by President Trump, wasn't seen by most as a good idea. By 47%-29%, they opposed doing that.

When it comes to tougher gun laws, the young people polled said by 7-1 that people who have been treated for mental illness should be barred from owning a firearm. By 2-1, they support banning semi-automatic weapons like the AR-15, similar to the gun used by the Parkland shooter. (Those under 18 were more likely to support banning semi-automatic weapons than those 18 and older.)

Those 18 and older said by 54%-33% that tightening gun laws and background checks would prevent more mass shootings in the United States. Those under 18 were less confident about that; they said by 47%-37% that taking those steps would work.

"You can make all the gun laws you want, but there's always going to be those people who go under the law and do illegal things, so how much are you preventing?" asks Tarena Marshall, 21, a student at the University of Southern Mississippi in Mendenhall. "You might stop a little of it but not all of it."

Among these young people, the March For Our Lives movement is viewed favorably by 40%, unfavorably by 11%, a net positive rating of 29 percentage points. Thirty percent have never heard of it. The National Rifle Association has a favorable-unfavorable rating of 30%-39%, a net negative rating of nine points. Eight percent have never heard of it.

"We have had massive student-led protests in the United States before," said David Farber, a history professor at the University of Kansas who studies social change. Until now, the largest were probably demonstrations in the spring of 1970 against a widening war in Vietnam. "Those student anti-war protests in 1970 were huge, but it seems likely that the marches planned on March 24 will be the largest student-led protests in the history of the United States, and certainly the largest inspired by high-school students."

Indeed, if 18% of those 13 to 24 years old were to participate personally in the marches — the percentage who said in the poll that they would — the demonstrations would draw more than nine million young people across the country.

Julian Perez, 23, a licensed practical nurse in Corpus Christi, Texas, suspects both sides of the debate will try to "spin" the student protests for their own purposes. "Some people were saying, 'It's great they care so much about politics,' and as soon as they hear something they don't like, it's, 'Well, you guys are just eating Tide pods; you shouldn't be talking."

But Angela Nemo, 21, of Philadelphia, is optimistic.

"A lot of times the older generations think they have everything together and figured out," she said, "but the younger generations bring fresh perspectives and definitely can make a difference."